Four planets are easily visible to the unaided eye: Venus, Mars, Jupiter and Saturn. In addition, Mercury, the innermost planet, may be glimpsed from time to time in the evening or morning twilight, but is usually too close to the Sun to be easily seen. Uranus and Neptune, the planets beyond Saturn, are too faint to see without some form of optical aid and even then appear as little more than points of light.
Where and when can I see the planets?
Planets move around the Sun in an orderly fashion, although it might not seem so as viewed from Earth. A planet's orbital period is inextricably linked to its distance from the Sun, as decreed by the laws of celestial motion: the more distant the planet, the longer it takes to complete one circuit. Venus, closer to the Sun than the Earth, completes one trip around the Sun in about seven and a half of our months, shining brilliantly in the evening and morning sky in turn. Being so startlingly bright, it is sometimes reported as a hovering UFO.
Twice on each orbital circuit Venus reaches a maximum angular separation from the Sun, its greatest elongation, once in the morning sky and once in the evening sky. At greatest elongation Venus lies between 45° and 47° from the Sun, and appears half illuminated through a telescope.
There are two other cardinal points that Venus reaches on each orbit: superior conjunction, when it is on the far side of the Sun from us and unobservable, and inferior conjunction, when it lies between Earth and Sun and is also unobservable (see diagram above right).